The invention relates to the moulding of metal parts using a low pressure moulding process and the mould structure per se.
U.S. Pat. No. 2,830,343 discloses a precision moulding process which uses a pattern formed of disposable expanded polystyrene. The expanded polystyrene pattern is surrounded by sand, with or without a binding agent. In the process disclosed in U.S. Pat. No. 2,830,343 the mould is gravity fed with a liquid metal.
In French Patent No. FR-A-2 163 455 and its corresponding U.S. Pat. No. 3,861,447, there is disclosed a precision moulding process which uses a pattern made of gasifiable expanded polystyrene. A sand mould free of a binding agent surrounds the pattern of gasifiable polystyrene. The sand is compacted around the pattern by subjecting the sand mould to a negative or subatmospheric pressure. In that pressure, like the aforementioned process described in U.S. Pat. No. 2,830,343, the mould is gravity fed. The pattern of gasifiable material, which because of its brittle nature is coated with a thermohardenable resin shell, is neither exactly positioned nor rigidly fashioned within the mould chamber. According to the teachings of this French Patent liquid metal is gravity fed into the mould chamber at a constant flow rate which is neither so high as to release polystyrene gas into the moulded part or so low as to cause a premature and uncontrolled evaporation of the polystyrene. To accomplish this constant feed, there is required a pouring basin or feed cone to control the required constant flow of the liquid metal. The result is a process which displays a low yield, the yield being defined as the ratio of the mass of useful metal to the overall metal mass used. A low yield is realized, for a significant portion of the overall metal mass ends up as solidified metal inside the pouring basin and non-useful appendages to the cast part. The useful metal is that metal which forms the cast part itself.
In French Patent No. FR-A-2 455 491 and its corresponding Canadian Patent No. 1,166,818, there is disclosed a foundry mould comprises of a shell and a core. According to this French Patent, the foundry mould is centered and immobilized in the middle of a mass of metal particles free of a binding agent and rigidified by a magnetic field. Molten metal is fed into the mould from the bottom up under low pressure.
The process of French Patent No. FR-A-2 455 491 is advantageous for in using low pressure, it is possible to monitor the flow of molten metal throughout the period of time it takes to fill the mould cavity. Additionally, the low pressure process substantially improves metal yield for the casting ducts are very short and surplus molten metal, which exists after a part is moulded and solidified, can be easily recovered in a ladel by introducing a sudden drop in pressure on the molten metal. Because the molten metal is pressure fed upwardly into the casting cavity, metal yield is maximized for excess metal remains fluid outside the moulding cavity and falls back into the ladle in response to the pressure drop. This process is also advantageous for it permits the controlled evacuation of gases and because it can be combined with a process for the controlled suctioning of gases to thereby prevent gas inclusion in the moulded part.
Thus, as exemplified by the aforementioned U.S. Pat. No. 2,830,343 and French Patent No. FR-A-2 163 455, it is known to use patterns of gasifiable material such as expanded polystyrene in a gravity fed mould. Such patterns can be used to produce accurately moulded parts for they can be made hollow and thin and shaped exactly as the desired part is to be shaped. However, because of their brittleness, patterns of gasifiable material have not been usable in foundrys which operate with the low pressure moulding process.